Quantitative studies of normal and abnormal human postural control mechanisms will continue using simultaneous center-of-force and head trajectory recordings. Controlled visual and galvanic vestibular inputs to the vestibulo-spinal control system will be employed in order to characterize time and frequency domain human postural control responses and to study visual-vestibular interactions. Gastrocnemius EMG recordings will be used with galvanic vestibular stimuli to study subjects with unilateral vestibular system deficits. Results will be used to further refine posturograhic techniques for clinical diagnostic evaluation of patients complaining of spatial disorientation.